Abstract:

A superconducting article is provided which includes a superconducting
tape assembly. The superconducting tape assembly includes a
superconducting tape layer, having one or more superconducting tapes, and
a high-permeability magnetic material layer coupled to the
superconducting tape layer. The high-permeability magnetic material layer
includes a high-permeability magnetic material which remains magnetically
soft at a critical temperature Tc of the superconducting tape, and
with presence of an ac magnetic field acting on the superconducting tape
assembly, re-magnetizes to divert at least a portion of a normal
component of the ac magnetic field therethrough, which reduces ac loss in
the superconducting tape layer by modifying the ac magnetic field
distribution within the superconducting tape of the superconducting tape
layer.

Claims:

1. A superconducting article comprising:a superconducting tape assembly,
the superconducting tape assembly comprising:a superconducting tape layer
comprising at least one superconducting tape; anda high permeability
magnetic material layer coupled to the superconducting tape layer,
wherein the high-permeability magnetic material layer comprises a
high-permeability magnetic material which remains magnetically soft at a
critical temperature Tc of the at least one superconducting tape,
and with presence of an ac magnetic field acting on the superconducting
tape assembly, re-magnetizes to divert at least a portion of a normal
component of the ac magnetic field therethrough, thereby reducing ac loss
in the superconducting tape layer by modifying ac magnetic field
distribution within the at least one superconducting tape.

2. The superconducting article of claim 1, wherein the normal component of
the ac magnetic field impinges the superconducting tape assembly
perpendicular to a main surface of the superconducting tape layer, and
the high-permeability magnetic material re-magnetizes to divert at least
a portion of the normal component of the ac magnetic field therethrough
in a direction parallel to the main surface of the superconducting tape
layer.

5. The superconducting article of claim 4, wherein the at least one
superconducting tape comprises a high temperature superconductor (HTS)
material, having a critical temperature Tc no less than about
77.degree. K.

6. The superconducting article of claim 4, wherein the superconducting
tape assembly is at least partial wound in a coil configuration, with the
ac magnetic field passing at least partially longitudinally through the
coil configuration, and the normal component of the ac magnetic field
being a radial component of the ac magnetic field.

5. The superconducting article of claim 1, wherein each superconducting
tape of the at least one superconducting tape comprises a substrate
supporting a superconducting region, and wherein the high-permeability
magnetic material layer is affixed to the superconducting tape layer with
the superconducting region of each superconducting tape of the at least
one superconducting tape being disposed adjacent and in opposing relation
to a main surface thereof.

8. The superconducting article of claim 1, wherein the high-permeability
magnetic material layer comprises a stack of at least two
high-permeability magnetic material tapes oriented substantially parallel
to the at least one superconducting tape of the superconducting tape
layer.

9. The superconducting article of claim 1, wherein the high-permeability
magnetic material layer is a first high-permeability magnetic material
layer coupled to a first main surface of the superconducting tape layer,
and wherein the superconducting tape assembly further comprises a second
high-permeability magnetic material layer coupled to a second main
surface of the superconducting tape layer, the second high-permeability
magnetic material layer comprising the high-permeability magnetic
material which remains magnetically soft at the critical temperature
Tc of the at least one superconducting tape, and wherein with
presence of an ac magnetic field acting on the superconducting tape
assembly, re-magnetizes to divert at least a portion of the normal
component of the ac magnetic field through the second high-permeability
magnetic material layer, the first and the second high-permeability
magnetic material layers reducing ac loss in the superconducting tape
assembly by modifying ac magnetic field distribution within the at least
one superconducting tape.

10. The superconducting article of claim 9, wherein each high-permeability
magnetic material layer of the first high-permeability magnetic material
layer and the second high-permeability magnetic material layer comprises
at least two high-permeability magnetic material tapes disposed in a
stack, and wherein each stack is affixed to the superconducting tape
layer via a respective adhesive foil.

11. The superconducting article of claim 1, wherein the superconducting
tape layer comprises one or more superconducting tapes disposed
substantially coplanar and affixed to a first main surface of the
high-permeability magnetic material layer, the superconducting tape layer
being a first superconducting tape layer, and wherein the superconducting
tape assembly further comprises a second superconducting tape layer
comprising one or more superconducting tapes disposed substantially
coplanar and affixed to a second main surface of the high-permeability
magnetic material layer.

12. The superconducting article of claim 11, wherein each superconducting
tape of the first superconducting tape layer and the second
superconducting tape layer includes a substrate supporting a
superconducting region, and wherein the superconducting tapes of the
first superconducting tape layer and the second superconducting tape
layer are coupled to the high-permeability magnetic material layer with
the superconducting region thereof disposed adjacent and in opposing
relation to a respective one of the first main surface and the second
main surface thereof.

13. The superconducting article of claim 11, wherein the superconducting
tape assembly is a stack of layers, and the high-permeability magnetic
material layer is a middle high-permeability magnetic material layer in
the stack of layers, and the superconducting tape assembly further
comprises an upper high-permeability magnetic material layer and a lower
high-permeability magnetic material layer, the upper high-permeability
magnetic material layer being affixed to the one or more superconducting
tapes of the first superconducting tape layer and the lower
high-permeability magnetic material layer being affixed to the one or
more superconducting tapes of the second superconducting tape layer, and
wherein each high-permeability magnetic material layer comprises the
high-permeability magnetic material which remains magnetically soft at
the critical temperature Tc of the superconducting tapes of the
first superconducting tape layer and the second superconducting tape
layer, and with presence of an ac magnetic field acting on the
superconducting tape assembly, re-magnetizes to divert at least a portion
of a normal component of the ac magnetic field through the high
permeability magnetic material layers, thereby reducing ac loss in the
first superconducting tape layer and the second superconducting tape
layer by modifying ac field distribution therein.

14. The superconducting article of claim 13, wherein the high-permeability
magnetic material layers are affixed to the superconducting tape layers
employing an adhesive foil, and wherein the superconducting tape layers
comprise high-temperature superconducting (HTS) conductors, each having a
critical temperature Tc not less than about 77.degree. K, and the
high-permeability magnetic material comprises a ferromagnetic glass
alloy.

15. A method of fabricating a superconducting article, comprising:forming
a superconducting tape assembly by:providing a superconducting tape layer
comprising at least one superconducting tape; andaffixing a
high-permeability magnetic material layer to the superconducting tape
layer, wherein the high-permeability magnetic material layer comprises a
high-permeability magnetic material which remains magnetically soft at a
critical temperature Tc of the at least one superconducting tape,
and with presence of an ac magnetic field acting on the superconducting
tape assembly, re-magnetizes to divert at least a portion of a normal
component of the ac magnetic field therethrough, thereby reducing ac loss
in the superconducting tape assembly by modifying ac magnetic field
distribution within the at least one superconducting tape.

16. The method of claim 15, wherein the affixing comprises stacking the
superconducting tape layer and the high-permeability magnetic material
layer with main surfaces thereof disposed in parallel, opposing relation,
and wherein the normal component of the ac magnetic field acts on the
superconducting tape assembly in a direction perpendicular to a main
surface of the superconducting tape layer, and the high-permeability
magnetic material layer is selected to re-magnetize to divert at least a
portion of the normal component of the ac magnetic field therethrough in
a direction parallel to the main surface of the superconducting tape
layer.

18. The method of claim 17, wherein the at least one superconducting tape
comprises a high-temperature superconductor (HTS) material, having a
critical temperature Tc not less than about 77.degree. K.

19. The method of claim 15, further comprising at least partially winding
the superconducting tape assembly in a coil configuration, and wherein
with the ac magnetic field passing at least partially longitudinally
through the coil configuration, the normal component of the ac magnetic
field is a radial component of the ac magnetic field.

20. The method of claim 15, wherein each superconducting tape comprises a
substrate supporting a superconducting region, and wherein the affixing
comprises affixing the high-permeability magnetic material layer to the
superconducting tape layer, with the superconducting region of each
superconducting tape of the at least one superconducting tape being
disposed adjacent and in opposing relation to a main surface thereof.

Description:

TECHNICAL FIELD

[0001]The present invention relates in general to superconducting
articles, and in particular, to a superconducting article (comprising a
superconducting tape assembly) and methods of fabrication thereof,
wherein reduced hysteretic ac losses are obtained by incorporating
therein one or more high permeability magnetic material layers.

BACKGROUND OF THE INVENTION

[0002]Superconductor materials have long been known and understood by the
technical community. Low-temperature (low-Tc) superconductors
exhibiting superconductive properties at temperatures requiring use of
liquid helium (4.2° K), have been known since about 1911. However,
it was not until somewhat recently that oxide-based high-temperature
(high-Tc) superconductors have been discovered. Around 1986, a first
high-temperature superconductor (HTS), having superconductive properties
at a temperature above that of liquid nitrogen (77° K) was
discovered, namely YBa2Cu3O7-x (YBCO), followed by
development of additional materials over the past 15 years, including
Bi2Sr2Ca2Cu3O10+y (BSCCO), and others. The
development of high-Tc superconductors has brought potentially,
economically feasible development of superconductors with liquid
nitrogen, rather than the comparatively more expensive cryogenic
infrastructure based on liquid helium.

[0003]Of the myriad of potential applications, the industry has sought to
develop use of such materials in the power industry, including
applications for power generation, transmission, distribution, and
storage. In this regard, it is estimated that the native resistance of
copper-based commercial power components is responsible for quite
significant losses in electricity, and accordingly, the power industry
stands to gain significant efficiencies based upon utilization of
high-temperature superconductors in power components such as transmission
and distribution power cables, generators, transformers, and fault
current interrupters. In addition, other benefits of high-temperature
superconductors in the power industry include an increase in one or two
orders of magnitude of power-handling capacity, significant reduction in
the size (i.e., footprint) of electric power equipment, reduced
environmental impact, greater safety, and increased capacity over
conventional technology. While such potential benefits of
high-temperature superconductors remain quite compelling, numerous
technical challenges continue to exist in the production and
commercialization of high-temperature superconductors on a large scale.

[0004]Among the many challenges associated with the commercialization of
high temperature superconductors, there remains the technical challenge
in the power industry of fabricating HTS cables and devices in such a way
that they operate with negligible alternating current (ac) losses. AC
current is the dominant form in most of the world's power cable-based
devices, and ac applications of HTS tapes operate with non-negligible
energy losses, with the energy escaping in the form of heat. This impacts
the efficiency of the system beyond mere energy loss since the heat
generated must be removed from the environment of the device.

[0005]Superconductors operate in the temperature range of
4°-85° K, far below ambient temperature (298° K).
Thus, superconductors require refrigeration, and refrigeration requires
continuous expenditure of energy, for example, if the heat caused by the
electrical current flowing in superconductor wires is at 77° K and
is dissipated at the rate of 1 Watt, then refrigerators must be supplied
with approximately 10-40 Watts of electrical power to dissipate that
generated heat. Absent this refrigeration, the superconductor would warm
itself to above its superconducting temperature and cease to operate as a
superconductor, thereby eliminating any advantage and, in particular,
providing worse performance than conventional copper conductors.

[0006]The heat generated must be eliminated to cost-effectively maintain
low temperatures required by the superconductor. A successful solution to
this problem would reduce operating costs by reducing the added cooling
energy needed. One significant problem with HTS tapes is that unwanted ac
magnetic fields are generated by the current flowing in the neighboring
HTS tapes which causes ac losses. Because the HTS tape material and
geometry is anisotropic, magnetic fields passing perpendicular to the
preferred direction generates significantly greater losses than those of
parallel fields.

[0007]In view of the foregoing, needs continue to exist in the art of
superconductors, and in particular, in the provision of commercially
viable superconducting tapes, method for forming the same, and power
components utilizing such superconductor tapes.

SUMMARY OF THE INVENTION

[0008]Accordingly, in one aspect, the present invention comprises a
superconducting article, which includes a superconducting tape assembly.
The superconducting tape assembly includes a superconducting tape layer,
comprising at least one superconducting tape, and a high-permeability
magnetic material layer coupled thereto. The high-permeability magnetic
material layer includes a high-permeability magnetic material which
remains magnetically soft at a critical temperature Tc of the at
least one superconducting tape, and with presence of an ac magnetic field
acting on the superconducting tape assembly, re-magnetizes to divert at
least a portion of a normal component of the ac magnetic field
therethrough, thereby reducing ac loss in the superconducting tape layer
by modifying ac magnetic field distribution within the at least one
superconducting tape.

[0009]In a further aspect, a method of fabricating a superconducting
article is provided. The method includes: forming a superconducting tape
assembly by: providing a superconducting tape layer comprising at least
one superconducting tape; and affixing a high-permeability magnetic
material layer to the superconducting tape layer, wherein the
high-permeability magnetic material layer comprises a high-permeability
magnetic material which remains magnetically soft at a critical
temperature Tc of the at least one superconducting tape, and with
presence of an ac magnetic field acting on the superconducting tape
assembly, re-magnetizes to divert at least a portion of a normal
component of the ac magnetic field therethrough, thereby reducing ac loss
in the superconducting tape assembly by modifying ac magnetic field
distribution within the at least one superconducting tape.

[0010]Further, additional features and advantages are realized through the
techniques of the present invention. Other embodiments and aspects of the
invention are described in detail herein and are considered a part of the
claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]The subject matter which is regarded as the invention is
particularly pointed out and distinctly claimed in the claims at the
conclusion of the specification. The foregoing and other objects,
features, and advantages of the invention are apparent from the following
detailed description taken in conjunction with the accompanying drawings
in which:

[0012]FIG. 1 illustrates one embodiment of a high temperature
superconducting tape to be employed in a superconducting tape assembly,
in accordance with an aspect of the present invention;

[0013]FIG. 2 illustrates one embodiment of a superconducting coil (e.g., a
solenoid), wound with a high-temperature superconducting tape, such as
illustrated in FIG. 1, with presence a magnetic field distribution as
illustrated and with ac losses in the coil to be reduced by the
superconducting tape assemblies disclosed herein, in accordance with an
aspect of the present invention;

[0014]FIG. 3A is a cross-sectional elevational view of one embodiment of a
low-hysteretic ac loss superconducting tape assembly, in accordance with
an aspect of the present invention;

[0015]FIG. 3B graphically depicts ac losses for a conventional high
temperature superconducting tape compared with a superconducting tape
assembly such as depicted in FIG. 3A plotted against the strength of the
ac magnetic field acting thereon in a direction perpendicular thereto, in
accordance with an aspect of the present invention;

[0016]FIG. 4A is a cross-sectional elevational view of an alternate
embodiment of a low-hysteretic ac loss superconducting tape assembly, in
accordance with an aspect of the present invention;

[0017]FIG. 4B graphically depicts ac losses of a conventional
multi-filamentary HTS conductor compared with a low-hysteretic ac loss
superconducting tape assembly such as depicted in FIG. 4A, plotted
against the strength of the ac magnetic field acting thereon in a
direction perpendicular thereto, in accordance with an aspect of the
present invention;

[0018]FIG. 5A is a cross-sectional elevational view of another embodiment
of a low-hysteretic ac loss superconducting tape assembly, in accordance
with an aspect of the present invention;

[0019]FIG. 5B graphically depicts ac losses of a single HTS conductor, a
multi-filamentary HTS conductor and a low-hysteretic ac loss
superconducting tape assembly such as depicted in FIG. 5A, plotted
against the strength of the ac magnetic field acting thereon in a
direction perpendicular thereto, in accordance with an aspect of the
present invention;

[0020]FIG. 6 is a partial cross-sectional view of one embodiment of a
power cable incorporating a low-hysteretic ac loss superconducting tape
assembly, in accordance with an aspect of the present invention;

[0021]FIG. 7 is a more detailed embodiment of a power cable incorporating
a low-hysteretic ac loss superconducting tape assembly, in accordance
with an aspect of the present invention;

[0022]FIG. 8 illustrates one embodiment of a power transformer
incorporating a low-hysteretic ac loss superconducting tape assembly, in
accordance with an aspect of the present invention;

[0023]FIG. 9 illustrates one embodiment of a rotating machine
incorporating a low-hysteretic ac loss superconducting tape assembly, in
accordance with an aspect of the present invention; and

[0024]FIG. 10 illustrates one embodiment of a power grid incorporating a
low-hysteretic ac loss superconducting tape assembly, in accordance with
an aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025]Referring to FIG. 1, the general layered structure of an HTS
conductor 100 is depicted which can be employed in a superconducting tape
assembly, in accordance with the present invention. The HTS conductor 100
includes a substrate 110, a buffer layer 111 overlying substrate 110, an
HTS layer 112, followed by a capping layer 114, (typically a noble metal
layer) and a stabilizer layer 116 (typically a non-noble metal). In the
embodiment depicted in FIG. 1, buffer layer 111, HTS layer 112, capping
layer 114 and stabilizer layer 116 are collectively referred to as the
superconducting region, which as illustrated, is disposed along one main
surface of substrate 110.

[0026]The substrate 110 is typically in a tape-like configuration, having
a high aspect ratio. For example, the width of the tape is generally on
the order of about 0.4-10 cm, and the length of the tape is typically at
least about 100 m, most typically greater than about 500 m. Accordingly,
the substrate may have an aspect ratio which is fairly high, on the order
of not less than 103, or even not less than 104. Certain
embodiments are longer, having an aspect ratio of 105 and higher. As
used herein, the term `aspect ratio` is used to denote the ratio of the
length of the substrate or tape to the next longest dimension, that is,
the width of the substrate or tape.

[0027]In one embodiment, the substrate is treated so as to have desirable
surface properties for subsequent deposition of the constituent layers of
the HTS tape. For example, the surface may be lightly polished to a
desired flatness and surface roughness. Additionally, the substrate may
be treated to be biaxially textured as is understood in the art, such as
by the known RABiTS (roll assisted biaxially textured substrate)
technique.

[0028]Turning to buffer layer 111, the buffer layer may be a single layer,
or more commonly, be made up of several films. Most typically, the buffer
layer includes a biaxially textured film, having a crystalline texture
that is generally aligned along crystal axes both in-plane and
out-of-plane of the film. Such biaxial texturing may be accomplished by
IBAD. As is understood in the art, IBAD is an acronym for Ion Beam
Assisted Deposition, a technique which may be advantageously utilized to
form a suitably textured buffer layer for subsequent formation of an HTS
layer having desirable crystallographic orientation for superior
superconducting properties. Magnesium oxide is a typical material of
choice for the IBAD film, and may be on the order or 50 to 500 Angstroms,
such as 50 to 200 Angstroms. Generally, the IBAD film has a rock-salt
like crystal structure, as defined and described in U.S. Pat. No.
6,190,752, which is incorporated herein by reference in its entirety.

[0029]The buffer layer may include additional films, such as a barrier
film provided to directly contact and be placed in between an IBAD film
and the substrate. In this regard, the barrier film may advantageously be
formed of an oxide, such as yttria, and functions to isolate the
substrate from the IBAD film. A barrier film may also be formed of
non-oxides such as silicon nitride and silicon carbide. Suitable
techniques for deposition of a barrier film include chemical vapor
deposition and physical vapor deposition including sputtering. Typical
thicknesses of the barrier film may be within a range of about 100-200
angstroms. Still further, the buffer layer may also include an
epitaxially grown film, formed over the IBAD film. In this context, the
epitaxially grown film is effective to increase the thickness of the IBAD
film, and may desirably be made principally of the same material utilized
for the IBAD layer such as MgO.

[0030]In embodiments utilizing an MgO-based IBAD film and/or epitaxial
film, a lattice mismatch between the MgO material and the material of the
superconducting layer exists. Accordingly, the buffer layer may further
include another buffer film, this one in particular implemented to reduce
a mismatch in lattice constants between the HTS layer and the underlying
IBAD film and/or epitaxial film. This buffer film may be formed of
materials such as YSZ (yttria-stabilized zirconia) strontium ruthenate,
lanthanum manganate, and generally, perovskite-structured ceramic
materials. The buffer film may be deposited by various physical vapor
deposition techniques.

[0031]While the foregoing has principally focused on implementation of a
biaxially textured film in the buffer stack (layer) by a texturing
process such as IBAD, alternatively, the substrate surface itself may be
biaxially textured. In this case, the buffer layer is generally
epitaxially grown on the textured substrate so as to preserve biaxial
texturing in the buffer layer. One process for forming a biaxially
textured substrate is the process known in the art as RABiTS (roll
assisted biaxially textured substrates), generally understood in the art.

[0032]High-temperature superconductor (HTS) layer 112 is typically chosen
from any of the high-temperature superconducting materials that exhibit
superconducting properties above the temperature of liquid nitrogen,
77° K Such materials may include, for example,
YBa2Cu3O7-x, Bi2Sr2Ca2Cu3O10+y,
Ti2Ba2Ca2Cu3O10+y, and
HgBa2Ca2Cu3O8+y. One class of materials includes
REBa2Cu3O7-x, wherein RE is a rare earth element. Of the
foregoing, YBa2Cu3O7-x, also generally referred to as
YBCO, may be advantageously utilized. The HTS layer 112 may be formed by
anyone of various techniques, including thick and thin film forming
techniques. Preferably, a thin film physical vapor deposition technique
such as pulsed laser deposition (PLD) can be used for a high deposition
rates, or a chemical vapor deposition technique can be used for lower
cost and larger surface area treatment. Typically, the HTS layer has a
thickness on the order of about 1 to about 30 microns, most typically
about 2 to about 20 microns, such as about 2 to about 10 microns, in
order to get desirable amperage ratings associated with the HTS layer
112.

[0033]Capping layer 114 and stabilizer layer 116 are generally implemented
for electrical stabilization, that is, to aid in prevention of HTS
burnout in practical use. More particularly, layers 114 and 116 aid in
continued flow of electrical charges along the HTS conductor in cases
where cooling fails or the critical current density is exceeded, and the
HTS layer moves from the superconducting state and becomes resistive.
Typically, a noble metal is utilized for capping layer 114 to prevent
unwanted interaction between the stabilizer layer(s) and the HTS layer
112. Typical noble metals include gold, silver, platinum, and palladium.
Silver is typically used due to its cost and general accessibility.
Capping layer 114 is typically made to be thick enough to prevent
unwanted diffusion of the components from stabilizer layer 116 into HTS
layer 112, but is made to be generally thin for cost reasons (raw
material and processing costs). Typical thicknesses of capping layer 114
range within about 0.1 to about 10.0 microns, such as 0.5 to about 5.0
microns. Various techniques may be used for deposition of capping layer
114, including physical vapor deposition, such as DC magnetron
sputtering.

[0034]According to a particular feature of an embodiment of the present
invention, stabilizer layer 116 is incorporated, to overlie the
superconducting layer 112, and in particular, overlie and directly
contact capping layer 114 in the embodiment shown in FIG. 1. Stabilizer
layer 116 functions as a protection/shunt layer to enhance stability
against harsh environmental conditions and superconductivity quench. The
layer is generally dense and thermally and electrically conductive, and
functions to bypass electrical current in case of failure in the
superconducting layer. Conventionally, such layers have been formed by
laminating a pre-formed copper strip onto the superconducting tape, by
using an intermediary bonding material such as a solder or flux. Other
techniques have focused on physical vapor deposition, typically,
sputtering. However, such application techniques are costly, and not
particularly economically feasible for large-scale production operations.
According to a particular feature of the embodiment, stabilizer layer 116
is formed by electroplating. According to this technique, electroplating
can be used to quickly build-up a thick layer of material on the
superconducting tape, and it is a relatively low cost process that can
effectively produce dense layers of thermally and electrically conductive
metals. According to one feature, the stabilizer layer is deposited
without the use of or reliance upon and without the use of an
intermediate bonding layer, such as a solder layer (including fluxes)
that have a melting point less than about 300° C.

[0035]Electroplating (also known as electrodeposition) is generally
performed by immersing the superconductive tape in a solution containing
ions of the metal to be deposited. The surface of the tape is connected
to an external power supply and current is passed through the surface
into the solution, causing a reaction of metal ions (Mz-) with
electrons (e.sup.-) to form a metal (M), wherein:

Mz-+ze.sup.-=M

[0036]Capping layer 114 functions as a second layer for deposition of
copper thereon. In the particular case of electroplating of stabilizer
metals, the superconductive tape is generally immersed in a solution
containing cupric ions, such as in a copper sulfate solution. Electrical
contact is made to capping layer 114 and current is passed such that the
reaction Cu2++2e-→Cu occurs at the surface of capping
layer 114. The capping layer 114 functions as the cathode in the
solution, such that the metal ions are reduced to Cu metal atoms and
deposited on the tape. On the other hand, a copper-containing anode is
placed in the solution, at which an oxidation reaction occurs such that
copper ions go into solution for reduction and deposition at the cathode.

[0037]In the absence of any secondary reactions, the current delivered to
the conductive surface during electroplating is directly proportional to
the quantity of metal deposited (Faraday's Law of Electrolysis). Using
this relationship, the mass, and hence thickness of the deposited
material forming stabilizer layer 116 can be readily controlled.

[0038]While the foregoing generally references copper, it is noticed that
other metals, including aluminum, silver, gold, and other thermally and
electrically conductive metals may also be utilized. However, it is
generally desirable to utilize a non-noble metal to reduce overall
materials cost for forming the superconductive tape.

[0039]While the foregoing description and FIG. 1 describes electroplating
to form stabilizer layer 116 along one side of the superconductive tape,
it is also noted that the opposite, major side of the superconductive
tape may also be coated, and indeed, the entirety of the structure can be
coated so as to be encapsulated. Those skilled in the art will note that
the above-description of HTS conductor 100 in FIG. 1 is provided by way
of example only. The superconducting tape assemblies described
hereinbelow may utilize any appropriate superconducting tape, for
example, any high-temperature superconducting tape having a critical
temperature Tc not less than about 77° K.

[0040]AC loss is a significant issue when superconductors are used for
applications where current or magnetic field vary in time. The reason for
this is that the motion of the flux lines, or change in the magnetic
field distribution, causes dissipation within the superconducting tape.
As shown in FIG. 2, a coil (e.g., a solenoid) 200, formed by winding a
superconducting tape 210, may have a magnetic field distribution B 220 as
illustrated. Near an upper end 201 and a lower end 202 of the coil, there
will be a radial component Br of the magnetic field that is normal
to the surface of superconducting tape 210. This normal component to the
superconducting tape would typically lead to an increased ac hysteretic
loss in the coil. Presented herein, however, is a superconducting tape
assembly which incorporates a magnetic screen, referred to herein as a
high-permeability magnetic material layer, to reduce adverse effects of
this transverse field, and thus reduce the net ac hysteretic loss of the
coil. More particularly, described hereinbelow is the reduction of ac
hysteretic losses in HTS superconductors (such as 2 G HTS
superconductors) by means of a superconducting tape assembly
incorporating high-permeability magnetic material screening of the
superconductor.

[0041]FIG. 3A is a cross-sectional elevational view of one embodiment of a
superconducting tape assembly, generally denoted 300, in accordance with
an aspect of the present invention. Superconducting tape assembly 300 is
shown to include a superconducting tape layer 310 sandwiched, in this
embodiment, between a first high-permeability magnetic material layer 320
and a second high-permeability magnetic material layer 321. In one
embodiment, these high-permeability magnetic material layers 320, 321 may
comprise thin foils of high-permeability ferromagnetic material which
functions to screen and redirect external ac magnetic field acting 330 on
superconducting tape layer 310, as illustrated by the re-directed
magnetic field lines. Specifically, the normal component of magnetic
field 330 is diverted, at least partially, by the high-permeability
magnetic material layers 320, 321, to a direction parallel to a main
surface of superconducting tape layer 310, thereby reducing ac loss in
the superconducting tape layer by modifying the ac magnetic field
distribution within the superconducting tape. Superconducting tape layer
310 may comprise one or more discrete superconducting tapes, depending on
the implementation.

[0042]As explained above, hysteretic ac losses in the superconducting tape
layer occur due to the motion of the flux lines in response to the
external ac field that causes dissipation. The dissipation power (or
power loss) is generally given by:

P=μ0fM(H)dH

[0043]Where integration is around a full magnetization cycle, P is the
power loss, μ0 is a constant, f is the frequency of the ac field,
M is the magnetization, which is a function of the applied magnetic field
H. By reducing the applied magnetic field H acting on the superconducting
tape, and changing the effective cross-section of the flux penetration,
as illustrated in FIG. 3A, it is possible to reduce ac power loss in the
superconducting tape layer.

[0044]As illustrated, once high-permeability magnetic material is placed
near one or both of the main surfaces of the superconducting tape layer,
the external field component normal to the tape layer is partially guided
away from the interior of the tape layer, towards its sides. Also, the
direction of the field has changed so that it is mostly parallel to the
main surfaces of the superconducting tape layer. Both effects work
together towards a significant reduction in ac field penetration of the
superconducting volume or region, resulting in the corresponding
reduction of ac loss. In one example, each high-permeability magnetic
material layer, 320, 321 (FIG. 3A) comprises a screen of two sheaths (or
thin foils) of high-permeability magnetic material. As used herein,
"high-permeability" refers to a magnetic permeability at room temperature
of at least 10,000.

[0045]In order to graphically illustrate the effect of the magnetic
material layers on ac loss reduction, the specific shielding
configurations described herein combine the following properties: a very
high magnetic permeability, also at 77° K, a small magnetic
hysteresis (low ferromagnetic ac loss), a high electrical resistivity
(low eddy current loss), good mechanical stability, and low cost.
Ferromagnetic metal glasses, produced by a number of companies, such as
Metglas, Inc., of Conway, S.C., possess the above-noted properties, and
thus are good candidates for use as magnetic shields in a manner
described herein. In the examples described below, a ferromagnetic glass
alloy, such as Metglas "SA01" or Metglas "2714A" may be used. In the
specific examples described herein, Metglas "SA01" was chosen for its
lower cost. The magnetic permeability of Metglas "SA01" at room
temperature is ≈45000. The foil used in the graphical comparisons
discussed herein had a thickness of 0.001 inch. Layers of magnetic
material were affixed with a Kapton adhesive foil to the superconducting
tape layer. Several configurations have been investigated (as illustrated
further in connection with FIGS. 4A-5B). In the first demonstration, a 40
mm long and 12 mm wide piece of HTS superconducting tape, with transport
critical current Ic=215 A, was stacked with two 40×12 mm
Metglas "SA01" foils on each side. AC loss with and without the magnetic
screens was measured, based on measurement of complex ac susceptibility
of the sample.

[0046]FIG. 3B illustrates experimental results for the superconducting
tape assembly of FIG. 3A. In FIG. 3B, ac loss in a single HTS conductor
(both without and with the magnetic screens made of a double layer of
"SA01" foil at each side) is illustrated. The ac loss, in watts per
meter, is plotted against the ac magnetic field strength applied
perpendicular to the HTS conductor, with field strength being an RMS
value in teslas. As noted in FIG. 3B, approximately a 10× loss
reduction is achieved near ≈100 G r.m.s. ac magnetic field
strength. The frequency of the ac field was 100 Hz in this example.

[0047]FIG. 4A depicts one embodiment of a multi-filament superconducting
tape assembly 400 configuration. In this example, five 2 mm wide and 40
mm long HTS conductors 410 with transport current Ic in the range of
22-25 A were affixed to a high-permeability magnetic material layer 420,
with the superconducting regions 411 of the conductors disposed adjacent
and in opposing relation to main surfaces of the high-permeability
magnetic material layer. As a specific example, the high-permeability
magnetic material layer may comprise a single sheath of Metglas "SA01"
foil, for example, 12 mm wide and 40 mm long. This high-permeability
magnetic material layer functions as the base for the multi-filamentary
superconducting tape assembly, as illustrated. In the embodiment of FIG.
4A, an ac magnetic field 430 is shown with a (normal) component acting on
the superconducting tape assembly from a direction perpendicular to a
main surface thereof, and more particularly, perpendicular to the
individual HTS conductors 410 of superconducting tape assembly 400. In
producing the superconducting tape assembly of FIG. 4A, the individual
superconducting tapes may be attached to the high-permeability magnetic
material layer using an adhesive foil, such as the above-noted Kapton
tape. In a specific example, the superconducting region 411 of the
superconducting tapes 410 comprises a YBCO superconductor facing the
high-permeability magnetic material layer 420.

[0048]FIG. 4B graphically illustrates the reduced ac loss in the
superconducting tape assembly of FIG. 4A compared with a conventional
multi-filamentary superconductor. In comparing the two approaches, five 2
mm wide HTS tapes were used, both with and without a high-permeability
magnetic material base layer of Metglas "SA01" alloy. As illustrated,
approximately an 8× loss reduction is achieved at low ac
(≈20 G) magnetic fields, and an ac field frequency of 100 Hz. As
shown, the maximum ac loss reduction in this configuration is achieved at
the lowest applied fields of approximately 10-20 Gauss.

[0049]FIG. 5A illustrates one embodiment of a multi-filamentary
superconducting tape assembly combining features of the superconducting
tape assembly of FIG. 3A, and the superconducting tape assembly of FIG.
4A. Specifically, the superconducting tape assembly, generally denoted
500, of FIG. 5A, is shown to comprise a first superconducting tape layer
501 and a second superconducting tape layer 502. Each superconducting
tape layer, in this example, includes two HTS conductors 510, having
superconducting regions 511 disposed adjacent to and in opposing relation
to a respective main surface of a middle high-permeability magnetic
material layer 520. In addition, the superconducting tape assembly 500
includes an upper high-permeability magnetic material layer 521 and a
lower high-permeability magnetic material layer 522. In this
configuration, the three high-permeability magnetic material layers, each
of which may comprise one or more films of ferromagnetic glass alloy,
such as the above-noted "SA01" provided by Metaglas, Inc., are used to
construct a tape assembly which provides both effects observed in the
above-described embodiments of FIGS. 3A & 4A. By way of further example,
each high-permeability magnetic material layer is assumed to comprise a
0.001 inch sheet of Metglas "SA01", and the superconducting tapes of the
superconducting tape layers are each assumed to comprise YBCO tape with a
transport critical current Ic=24-25 A.

[0050]As illustrated in FIG. 5B, the ac power loss of a multi-filamentary
conductor made of four 2 mm wide HTS tapes in comparison with a single 8
mm wide HTS tape nets a 4× reduction in ac losses, while an
approximately 850× reduction in ac loss is achieved employing a
superconducting tape assembly such as depicted in FIG. 5A. In the
illustrated example, the superconducting tape assembly of FIG. 5A is
again assumed to be made of four 2 mm wide HTS tapes arranged in the two
superconducting tape layers, along with the three high-permeability
magnetic material layers, each comprising one or more ferromagnetic glass
alloy foils, such as the above-discussed Metglas "SA01" alloy. An
approximately 200× ac loss reduction is achieved with magnetic
screening at 100 Gauss r.m.s. ac field, and the total reduction obtained
with striation of the tape, plus the magnetic screening described herein,
is approximately 850×, with the frequency of the field being 100
Hz.

[0051]To summarize, combining three high-permeability magnetic material
layers of "SA01" alloy in the filamentary tape assembly of FIG. 5A
produces over 200× reduction in ac loss. This is in addition to the
approximately 4× reduction due to division of the superconductor
into filaments, compared with an 8 mm tape. Thus, an approximately
850× ac loss reduction is obtained using the combination of
striation and magnetic screening in a superconducting tape assembly,
which is very significant. The superconducting tape assemblies described
herein advantageously allow reduction in ac magnetization losses of an
HTS superconducting tape by over two orders of magnitude using a
technique of laminating the tape surface(s) with a soft magnetic alloy.
In one example, Kapton adhesive tape may be used to combine the HTS
superconductors with the magnetic foils. Longer length embodiments may
utilize a continuous lamination process of the HTS tapes with the
magnetic material.

[0052]Various superconducting article configurations are depicted in FIGS.
6-10 and described below. The superconducting tape assembly presented
herein may be employed in one or more of these articles, particularly
where there is a perpendicular magnetic field component acting on the
superconducting tape assembly. For example, a superconducting tape
assembly such as described herein may be used in one or more of the
superconducting articles described below if a portion or all of the
superconducting tape within the superconducting article experiences an ac
magnetic field with a component normal to the superconducting tape
surface. In such a case, the superconducting tape assembly described
herein provides reduced ac loss in the superconducting tape layer by
modifying ac magnetic field distribution within the superconducting tape.

[0053]FIGS. 6 and 7 illustrate implementation of a superconducting tape
assembly such as described herein within a superconducting article,
namely a power cable 600. FIG. 6 illustrates several power cables 620
extending through an underground conduit 610, which may be a plastic or
steel conduit, and illustrates ground 630 for clarity. As is shown,
several power cables 620 may run through conduit 610.

[0054]FIG. 7 depicts a particular structure of a power cable 620. In order
to provide cooling to maintain the superconductive power cable in a
superconducting state, liquid nitrogen is fed through the power cable
through LN2 duct 700. One or a plurality of HTS tapes 702 is/are provided
so as to cover the duct 700. The tapes may be placed onto the duct 700 in
a helical manner, spiraling the tape about the duct 700. Further
components include a copper shield 704, a dielectric tape 706 for
dielectric separation of the components, a second HTS tape 708, a copper
shield 710 having a plurality of centering wires 712, a second, larger
LN2 duct 714, thermal insulation 716, provided to aid in maintaining a
cryogenic state, a corrugated steel pipe 718 for structural support,
including skid wires 720, and an outer enclosure 722.

[0055]FIG. 8 illustrates schematically a power transformer 800 having a
central core 830 around which a primary winding 810 and a secondary
winding 820 are provided. As shown, FIG. 8 is schematic in nature, and
the actual geometric configuration of the transformer may vary as is well
understood in the art. As illustrated, the transformer includes the basic
primary and secondary windings. In this regard, in the embodiment shown
in FIG. 9, primary winding 810 has a higher number of coils than
secondary winding 820, representing a step-down transformer which reduces
voltage of an incoming power signal. In reverse, provision of a fewer
number of coils in the primary winding relative to the secondary winding
provides a voltage step-up. In this regard, typically step-up
transformers are utilized in power transmission substations to increase
voltage to high voltages to reduce power losses over long distances,
while step-down transformers are integrated into distribution substations
for later stage distribution of power to end users. At least one, and
possibly both, of the primary and secondary windings may comprise
superconducting tape assemblies, in accordance with the foregoing
description.

[0056]FIG. 9 depicts the basic structure of a generator. The generator
includes a rotor 900 that is driven as is known in the art, such as by a
turbine. Rotor 900 includes high-intensity electromagnets, which are
formed of rotor coils 910 that form the desired electromagnetic field for
power generation. The generation of the electromagnetic field generates
power in the stator 920, which comprises at least one conductive winding
930. According to a particular feature of the embodiment, the rotor coils
and/or the stator winding comprises a superconducting tape assembly, such
as described herein. Low-loss superconductors used in the stator windings
generally substantially reduce hysteresis losses.

[0057]Turning to FIG. 10, a basic schematic of a power grid 1000 is
provided. Fundamentally, power grid 1000 includes a power plant 1010,
typically housing a plurality of power generators, and transmission lines
1012, to deliver power to a transmission substation 1020. Transmission
substation 1020 contains generally a bank of step-up power transformers
1021, which are utilized to step-up voltage of the generated power.
Typically, power is generated at a voltage level on the order of
thousands of volts, and the transmission substation functions to step-up
voltages to the order of 100,000 to 1,000,000 volts in order to reduce
line losses. Typical transmission distances are on the order of 50 to
1,000 miles, and power is carried along those distances by power
transmission cables 1022, 1026. Power transmission cables 1022, 1026 are
routed to a plurality of power substations 1030 (only one of which is
shown in FIG. 10). The power substations contain generally a bank of
step-down power transformers 1031, to reduce the transmission level
voltage from the relatively high values to distribution voltages,
typically less than about 10,000 volts. A plurality of further power
substations may also be located in a grid-like fashion, provided in
localized areas for localized power distribution to end users. However,
for simplicity, only a single power substation is shown, noting that
downstream power substations may be provided in series. The distribution
level power is then transmitted along power distribution cables 1032 to
end users 1040, which include commercial end users as well as residential
end users. It is also noted that individual transformers may be locally
provided for individual or groups of end users. According to a particular
feature, at least one of the generators provided in the power plant, the
transformers in the transmission substation, the power transmission
cable, the transformers provided in the power substation, and the power
distribution cables contain a superconducting tape assembly, in
accordance with the present description.

[0058]Although preferred embodiments have been depicted and described in
detail herein, it will be apparent to those skilled in the relevant art
that various modifications, additions, substitutions and the like can be
made without departing from the spirit of the invention and these are
therefore considered to be within the scope of the invention as defined
in the following claims.